This invention relates to polymerizable dispersants, and more specifically, to phosphate-based ester dispersants that are effective in nonaqueous filled suspensions due to polymerizable groups that strengthen the composite after polymerization.
High mechanical forces are required to incorporate solids into liquid media. To reduce the required dispersing forces, it is known to use dispersants, which facilitate the incorporation. Dispersants have been used in paints, molding compositions, coatings, plastic mixtures, printing inks, synthetic resin systems, and the like. The solids to be incorporated into the liquid media include, for example, pigments, fillers and fibers.
U.S. Pat. No. 5,151,218 discloses phosphoric acid ester dispersants of various structures. Recognized therein is the limited usefulness of certain structures as a dispersant in a broad number of applications. If a compound is not sufficiently compatible with the system, inadequate dispersion occurs due to insufficient interaction with the surrounding medium, and potentially precipitation phenomena with associated dulling, spotting and increased viscosity can occur. There is thus a need to develop dispersants that are broadly useful for wide-ranging applications.
In some applications, it is desirable to maximize the amount of solid in the suspension. For example, in a synthetic resin system, it may be desirable to limit the amount of the polymerizable resin and maximize the amount of filler, fiber or pigment material. The main factor limiting the volume fraction (load) of the solid in highly filled suspensions is particle-particle interactions. Dispersants, through their ability to reduce interactions between particles, can improve the flow (reduce the viscosity) of the suspension, thereby allowing a higher load. Dispersants in non-aqueous systems reduce particle interactions by a steric stabilization mechanism. A layer of the dispersant is adsorbed on the surface of the particles keeping them apart from one another, reducing the viscosity. The dispersant structure must contain a chain that allows for steric stabilization in the resin and it also must be strongly adsorbed on the particle surface. There is thus a further need to provide a dispersant that will be effective with a non-aqueous, highly filled suspension containing polymerizable resin.
The present invention provides a polymerizable monophosphate ester dispersant, a nonaqueous, highly-filled suspension containing the polymerizable monophosphate ester dispersant, and a method for forming cured articles by copolymerization of the resin matrix and dispersant in the suspension.
The dispersant of the present invention comprises a phosphate group, polyester group and polymerizable end group. The dispersant is trifunctional in nature. An exemplary dispersant of the present invention is a monophosphate ester according to the formula 
wherein R is a (meth)acrylate group radical or vinyl radical, and wherein n represents the number of units of caprolactone. Further exemplary embodiments include dispersants in which R is one of the following (meth)acrylate group radicals: oxyethyl methacryloyl-, oxyethyl acryloyl-, polyoxypropyl methacryloyl-, glyceryl dimethacryloyl-, and dipentaerythritol pentaacryloyl-. The inclusion of the dispersant, generally in an amount up to about 5 wt. %, in a suspension of resin and filler enables an increase in the filler loading, which results in reduced shrinkage, a lower coefficient of thermal expansion and generally improved physical properties in an article cured therefrom. These and other objects and advantages of the present invention shall become more apparent from the description of the preferred embodiments and the examples.
The present invention provides a class of phosphate based dispersants that are effective with nonaqueous, highly-filled suspensions. The dispersant is a trifunctional molecule that binds to the surface of a solid particulate material, reduces particle interactions by steric stabilization, and is copolymerizable with a resin matrix. Bonding with the surface of the solid, inorganic particulate material is accomplished with an anchoring phosphate group or ion. The phosphate ion has an affinity for the inorganic material. A spacer group, for example a polyester group, penetrates into the resin to promote steric stabilization. A polymerizable end group, either a monomer or copolymer, for example vinyl, methacrylate or acrylate, enables the dispersant to be copolymerized with the resin matrix. The presence of these groups in the dispersants of the present invention result in excellent compatibility with a broad range of resin systems, particularly (meth)acrylate-based (meaning methacrylate or acrylate) resin systems. Inclusion of the dispersants of the present invention into filled resin formulations will result in cured articles having lower shrinkage upon polymerization, lower coefficient of thermal expansion, and generally improved properties as compared to a formulation without the dispersant. In addition, copolymerization of the dispersant with the resin matrix provides improved bonding between solid particles and the resin matrix.
In one aspect of the present invention, inclusion of a novel dispersant in (meth)acrylate-based resin dental composite formulations results in increased filler loading and decreased viscosity, which after curing of the suspension provides a dental restorative with reduced shrinkage, a lower coefficient of thermal expansion and generally improved physical properties.
The dispersants of the present invention preferably comprise about 5 wt. % or less of the highly-filled suspensions, such as a dental composite paste. To obtain good uniformity of distribution of the dispersant in the final suspension, the dispersant is first mixed with the resin, followed by the slow addition of the solid particulate material, such as the inorganic glass fillers used in dental composites.
An exemplary dispersant of the present invention is a monophosphate ester of the formula 
wherein R is a (meth)acrylate group radical or vinyl radical, and wherein n represents the number of units of caprolactone.
The presence of a carboxylic acid ester group of the dispersant results in excellent compatibility with (meth)acrylate-based resin systems. In exemplary embodiments, the dispersants of the present invention are monophosphate esters according to the above formula where R is one of the following (meth)acrylate group radicals:
Compound 1: R=oxyethyl methacryloyl 
Compound 2: R=oxyethyl acryloyl 
Compound 3: R=polyoxypropyl methacryloyl 
wherein m represents the number of units of oxypropyl. 
Compound 5: R=dipentaerythritol pentaacryloyl 
Compound 6: R=polyoxyethyl methacryloyl 
wherein m represents the number of units of oxyethyl.
Each of Compounds 1-6 may be prepared in two steps. In the first step, the hydroxy functional methacrylate is condensed with caprolactone under ring-opening polymerization conditions in the presence of catalytic amounts of SnCl2 to prepare a polyester. In the second step, the polyester is reacted with polyphosphoric acid (117.5% concentration) at 65xc2x0 C. to produce the phosphoric acid (monophosphate) ester. By way of example, the reaction sequence is shown below for the preparation of the hydroxyethyl methacrylate (HEMA) derivative Compound 1; it should be noted that substantially the same steps and conditions would be utilized to prepare any of Compounds 1-6: 
HEMA, MW=130.14 Caprolactone, MW=114.14
Polycaprolactone-modified HEMA 
Compound 1: Polycaprolactone-modified HEMA Phosphate